Swift typeclass example

num.swift

protocol Num {
  class func zero() -> Self
  func succ() -> Self
  func add(y: Self) -> Self
  func multiply(y: Self) -> Self
}

extension Int32: Num {
  static func zero() -> Int32 { return 0 }
  func succ() -> Int32 { return self + 1 }
  func add(y: Int32) -> Int32 { return self + y }
  func multiply(y: Int32) -> Int32 { return self * y }
}

func sum<A : Num>(xs: Array<A>) -> A {
  return xs.reduce(A.zero(), { $0.add($1) })
}

let l : Array<Int32> = [1,2,3]
println(sum(l))